Diabetes is a complex group of diseases with a variety of causes. People with diabetes have high blood glucose, also called high blood sugar or hyperglycemia. Diabetes is a disorder of metabolism that develops when the body doesn't produce enough insulin or is not able to use insulin effectively, or both. Insulin is made in the pancreas, an organ located behind the stomach. The pancreas contains clusters of cells called islets. Beta cells within the islets make insulin and release it into the blood.
Type 1 diabetes is caused by a lack of insulin due to the destruction of insulin-producing beta cells in the pancreas. In type 1 diabetes—an autoimmune disease—the body's immune system attacks and destroys the beta cells. Normally, the immune system protects the body from infection by identifying and destroying bacteria, viruses, and other potentially harmful foreign substances. But in autoimmune diseases, the immune system attacks the body's own cells. In type 1 diabetes, beta cell destruction may take place over several years, but symptoms of the disease usually develop over a short period of time.
Type 2 diabetes mellitus (T2DM) is a complex and heterogeneous disorder involving many physiological risk factors and genetic susceptibility factors. It has long been noticed that humans, non-human primates, and cats are susceptible to spontaneous development of T2DM characterized by islet amyloid deposits while spontaneous T2DM is rare in rodents and pigs. One hypothesis is that formation of islet amyloid from islet amyloid polypeptide (IAPP) is a pathogenic factor for β-cell degeneration and apoptosis, which gradually causes T2DM (Matveyenko and Butler, 2006, ILAR J 47(3): 225-33). The major component of islet amyloid is formed by fibrils of IAPP, a 37-amino acid monomeric polypeptide synthesized by pancreatic β-cells. The residues 20-29 of IAPP are the most critical region for the amyloidogenic properties (Westermark et al., 1990, Proc Natl Acad Sci USA 87(13): 5036-40). In humans, monkeys and cats, IAPP is prone to form toxic amyloid aggregates, whereas IAPP from rodents and pigs is more refractory to amyloid formation (Zhang et al., 2011, FEBS Lett 585(1): 71-7). Furthermore, a susceptibility variant of the human IAPP gene (S20G) has been found to be associated with early onset Type 2 diabetes (T2DM) in Asian populations (Morita et al., 2011).
Animal models, such as rodents, rabbits, dogs, cats, pigs and primates, have been used extensively in T2DM research, investigating disease pathogenesis and testing preventions and therapies. With the advances in transgenic technologies, many transgenic, complete knockout and conditional knockout mice have been generated for diabetic research. Recently, gene knockout rats have been generated with Zinc-Finger Nucleases (ZFNs) for diabetic research (Geurts et al., 2009, Science 325(5939): 433). Nevertheless, rodents are still considered to be inadequate to reflect the pathogenesis of T2DM in humans. Thus, most clinical and translational research, including testing of prevention strategies and drugs, require large animal models such as pigs.
Several mouse models expressing human IAPP (hIAPP) have been created to study the pathogenesis of IAPP-associated T2DM. These hIAPP transgenic mice were generated by targeting the expression of IAPP to pancreatic β-cells using the rat insulin 2 promoter (RIP2) (Couce et al., 1996, Diabetes 45(8): 1094-101; Janson et al., 1996, Proc Natl Acad Sci USA 93(14): 7283-8). Spontaneous onset T2DM was observed in hIAPP homozygous mice. However, hIAPP hemizygous mice do not develop diabetes unless they have obese background or are treated with growth hormone. Several islet pathological phenotypes such as intra- and extracellular amyloid deposits, increased β-cell apoptosis, and decreased β-cell mass were observed in diabetic hIAPP mice. Based on the studies of hIAPP mouse models, dose dependent hIAPP expression is suggested to be a critical factor for β-cell toxicity and T2DM pathogenesis (Matveyenko and Butler, 2006, ILAR J 47(3): 225-33).
A transgenic rat model for human IAPP (HIP rat) was generated for studying T2DM pathogenesis associated with islet amyloid formation (Butler et al., 2004, Diabetes 53(6): 1509-16). Similar to hIAPP mice, the pathogenic process depended on the dose of hIAPP expression and the homozygous HIP rat developed diabetes between 5 and 10 months of age, displaying island amyloid and an approximately 60% deficit in β-cell mass. Thus, both mouse and rat diabetic hIAPP models developed islet pathology related to that in humans. Nevertheless, the rodent hIAPP models cannot fully address the pathogenic process of T2DM associated with islet amyloid formation by IAPP aggregation. Although IAPP pathology was reported to induce insulin resistance, insulin resistance was not observed in the rodent IAPP models.
One major weakness of previous mouse or rat IAPP models is that these animal models are still expressing their own IAPP gene. Previous study have shown that the presence of none aggregate IAPP peptide (e.g. the porcine IAPP peptide) can delay/prevent amyloid formation. This suggests that the discrepancy of diabetic phenotypes observed in the previous rodent IAPP models might be due to compensatory effects and/or anti-aggregation effects from endogenous IAPP. Thus, new animal models generated by targeted replacement of the endogenous IAPP gene (knockout) with amyloidogenic form of hIAPP (knockin) should represent a better model for studying IAPP's role in islet pathology of T2DM patients.
Pigs have been favoured as an animal model for T2DM due to the anatomical and physiological similarities of the pig and human pancreas and islets. Several pig models of chemically induced diabetes have been generated by streptozotocin administration. Most importantly, the generation of transgenic (GM) pigs for diabetic research has now become possible by cloning based on somatic cell nuclear transfer (SCNT). The first GM pig model for T2DM was a transgenic pig expressing a dominant negative mutant form of the glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPRdn) in pancreatic cells. It was generated in the laboratory of Eckhard Wolf in Munich (Renner et al., 2010, Diabetes 59(5): 1228-38). Similar to the GIPRdn mice, the GIPRdn pigs exhibited significant oral glucose tolerance reduction and β-cell proliferation reduction by the age of 11 weeks. Glucose control deterioration and reduction of β-cell mass were observed in the GIPRdn pigs with increasing age (Renner et al., 2010, Diabetes 59(5): 1228-38). Another successful GM pig model of diabetes was a transgenic pig expressing a dominant negative mutant hepatocyte nuclear factor 1α (HNF1AP291fsinsC) gene. The HNF1AP291fsinsC pigs developed diabetes at the age of 20-196 days, characterized by non-fasting blood glucose higher than 200 mg/dl, as well as small and irregularly formed Langerhans Islets (Umeyama et al., 2009, Transgenic Res 18(5): 697-706). The successful modeling of human diabetes in the GIPRdn and HNF1AP291fsinsC pigs has provided proof of concept for the future development of other GM pig models for the human disease.